20 research outputs found
Tunneling behavior of Ising and Potts models in the low-temperature regime
We consider the ferromagnetic -state Potts model with zero external field
in a finite volume and assume that the stochastic evolution of this system is
described by a Glauber-type dynamics parametrized by the inverse temperature
. Our analysis concerns the low-temperature regime ,
in which this multi-spin system has stable equilibria, corresponding to the
configurations where all spins are equal. Focusing on grid graphs with various
boundary conditions, we study the tunneling phenomena of the -state Potts
model. More specifically, we describe the asymptotic behavior of the first
hitting times between stable equilibria as in probability,
in expectation, and in distribution and obtain tight bounds on the mixing time
as side-result. In the special case , our results characterize the
tunneling behavior of the Ising model on grid graphs.Comment: 13 figure
Delay performance in random-access grid networks
We examine the impact of torpid mixing and meta-stability issues on the delay
performance in wireless random-access networks. Focusing on regular meshes as
prototypical scenarios, we show that the mean delays in an toric
grid with normalized load are of the order . This
superlinear delay scaling is to be contrasted with the usual linear growth of
the order in conventional queueing networks. The intuitive
explanation for the poor delay characteristics is that (i) high load requires a
high activity factor, (ii) a high activity factor implies extremely slow
transitions between dominant activity states, and (iii) slow transitions cause
starvation and hence excessively long queues and delays. Our proof method
combines both renewal and conductance arguments. A critical ingredient in
quantifying the long transition times is the derivation of the communication
height of the uniformized Markov chain associated with the activity process. We
also discuss connections with Glauber dynamics, conductance and mixing times.
Our proof framework can be applied to other topologies as well, and is also
relevant for the hard-core model in statistical physics and the sampling from
independent sets using single-site update Markov chains
Experimental results of the laserwire emittance scanner for LINAC4 at CERN
Within the framework of the LHC Injector Upgrade (LIU), the new LINAC4 is
currently being commissioned to replace the existing LINAC2 proton source at
CERN. After the expected completion at the end of 2016, the LINAC4 will
accelerate H- ions to 160 MeV. To measure the transverse emittance of the H-
beam, a method based on photo-detachment is proposed. This system will operate
using a pulsed laser with light delivered via an optical fibre and subsequently
focused through a thin slice of the H- beam. The laser photons have sufficient
energy to detach the outer electron and create H0/e- pairs. In a downstream
dipole, the created H0 particles are separated from the unstripped H- ions and
their distribution is measured with a dedicated detector. By scanning the
focused laser across the H- beam, the transverse emittance of the H- beam can
be reconstructed. This paper will first discuss the concept, design and
simulations of the laserComment: Presented at the International Conference on Laser Applications at
Accelerators, LA3NET 2015; Submitted to Nucl. Instr. and Meth. in Phys. Res.
section
Commissioning of the CERN LINAC4 Wire Scanner, Wire Grid and Slit-Grid Monitors at 3 and 12 MeV
The CERN LINAC4 has been commissioned up to 12 MeV. The H^{−} beam transverse profile distributions were measured by both wire grids and wire scanners. A slit-grid system located on a temporary diagnostics bench was used to characterize the transverse emittance during the two different stages of commissioning: at the exit of the RFQ (3 MeV) and DTL1 (12 MeV). The wire signal is a balance between the negative charge deposited by the stripped electrons from the H^{−} and the charge lost due to secondary emission. Optimal settings were found for the repelling plates used to suppress secondary emission, which were confirmed by electromagnetic simulations. In addition, suppression of the secondary emission due to the beam space charge was observed. The benefit of changing the wire scanner geometry in order to minimize the cross-talk between horizontal and vertical wires and the observation of thermionic emission on carbon wires are also discussed
Hitting times asymptotics for hard-core interactions on grids
We consider the hard-core model with Metropolis transition probabilities on finite grid graphs and investigate the asymptotic behavior of the first hitting time between its two maximum-occupancy configurations in the low-temperature regime. In particular, we show how the order-of-magnitude of this first hitting time depends on the grid sizes and on the boundary conditions by means of a novel combinatorial method. Our analysis also proves the asymptotic exponentiality of the scaled hitting time and yields the mixing time of the process in the low-temperature limit as side-result. In order to derive these results, we extended the model-independent framework in [27] for first hitting times to allow for a more general initial state and target subset.
Keywords: hard-core model; hitting times; Metropolis Markov chains; finite grid graphs; mixing times; low temperature
Real-time PCR analysis of genes encoding tumor antigens in esophageal tumors and a cancer vaccine
Tumor antigens are the primary target of therapeutic
cancer vaccines. We set out to define and compare the expression
pattern of tumor antigen genes in esophagus carcinoma biopsies and
in an allogeneic tumor lysate-based cancer vaccine, MelCancerVac®.
Cells used for vaccine production were treated with the DNA methyltransferase
inhibitor 5-aza-2'-deoxycytidine (5-aza-CdR) to determine whether
this treatment could improve the profile of tumor antigen genes
expressed in these cells. In addition, the presence of MAGE-A tumor
antigen protein was evaluated in the purified tumor cell lysate
used in the production of the vaccine. Quantitative PCR was used
to assay 74 tumor antigen genes in patients with squamous cell carcinoma
of the esophagus. 81% (13/16) of tumors expressed more
than five cancer/testis (CT) antigens. A total of 96 genes
were assayed in the tumor cell clone (DDM1.7) used to make tumor
cell lysate for vaccine preparation. Gene expression in DDM1.7 cells
was compared with three normal tissues; 16 tumor antigen genes were induced
more than ten-fold relative to normal tissues. Treatment with 5-aza-CdR
induced expression of an additional 15 tumor antigens to a total
of 31. MAGE-A protein was detected in cell lysate by Western blot
at an estimated concentration of 0.2 µg/ml or
0.01% of the total protein
Longitudinal Beam Profile Measurements in Linac4 Commissioning
Linac4, the future 160 MeV H^{−} injector to the CERN Proton Synchrotron Booster, is presently under construction at CERN as a central step of the planned upgrade of the LHC injectors. The Linac front-end, composed of a 45 keV ion source, a Low Energy Beam Transport (LEBT), a 352.2 MHz Radio Frequency Quadrupole (RFQ) and a Medium Energy Beam Transport (MEBT) housing a beam chopper, has been installed and commissioned. Precise measurements of the longitudinalμbunch profiles of ion beams were possible with the help of a Bunch Shape Monitor (BSM) developed at INR Moscow. These were crucial for the successful commissioning of the three RF buncher cavities mounted along the MEBT and well complemented with higher precision the information provided in parallel by spectrometer measurements
Transverse Beam Profile Measurements in the Linac4 Medium Energy Beam Transport
Linac4 is a 160 MeV H^{−} linear accelerator presently under construction at CERN. It will replace the present 50 MeV proton Linac2 as injector of the proton accelerator complex as part of a project to increase the LHC luminosity. The Linac4 front-end, composed of a 45 keV ion source, a Low Energy Beam Transport (LEBT), a 352.2 MHz Radio Frequency Quadrupole (RFQ) which accelerates the beam to 3 MeV and a Medium Energy Beam Transport (MEBT) housing a beam chopper, has been commissioned in the Linac4 tunnel. The MEBT is composed of three buncher cavities and 11 quadrupole magnets to match the beam from the RFQ to the next accelerating structure (DTL) and it includes two wire scanners for beam profile measurement. In this paper we present the results of the profile measurements and we compare them with emittance measurements taken with a temporary slit-and-grid emittance measurement device located after the MEBT line
The role of brain MRI in mitochondrial neurogastrointestinal encephalomyopathy.
Leukoencephalopathy is a hallmark of mitochondrial neurogastrointestinal encephalomyopathy (MNGIE) a devastating disorder characterized by ptosis, ophthalmoparesis, gastrointestinal dysfunction and polyneuropathy. To characterize MNGIE-associated leukoencephalopathy and to correlate it with clinical, biochemical and molecular data, four MNGIE patients with heterogeneous clinical phenotypes (enteropathic arthritis, exercise intolerance, CIDP-like phenotype and typical presentation) were studied by magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS). Diffusion weighted imaging (DWI) with apparent diffusion coefficient (ADC) maps were also obtained. In two patients we also investigated the role of brain MRI in monitoring the evolution of leukoencephalopathy by performing follow-up imaging studies at an interval of one and two years. The extension and distribution of leukoencephalopathy were not clearly linked with age, phenotype or disease severity, and did not seem to be related to TYMP mutations, enzyme activity or pyrimidine levels. In the studied patients MRS revealed reduced N-acetyl-aspartate and increased choline signals. Although DWI appeared normal in all patients but one, ADC maps always showed moderate increased diffusivity. Leukoencephalopathy worsened over a two-year period in two patients, regardless of the clinical course, indicating a lack of correlation between clinical phenotype, size and progression of white matter abnormalities during this period. Brain MRI should be considered a very useful tool to diagnose both classical and atypical MNGIE. Serial MRIs in untreated and treated MNGIE patients will help to establish whether the leukoencephalopathy is a reversible condition or not